1. Field of the Invention
This invention relates to infrared and/or ultraviolet radiation absorbing soda-lime-silica glass composition suitable for architectural and automotive glazing applications. The glass article has high redox values greater than 0.38 which makes the article difficult to melt from batch and refine in a horizontal continuous furnace with a melter and refiner. The melter of the furnace is heated at least partially from overhead fossil fuel fired burners and the melted glass moves and advances along as a pool of molten glass maintained in the melter and through the refiner and to a glass forming operation.
2. Technical Considerations
Infrared and ultraviolet radiation absorbing colored glass substrates have a variety of different applications. In particular, such glasses may be used by architects to glaze buildings and by vehicle designers as automotive windows. Besides providing an aesthetically pleasing color, these glasses may also provide enhanced solar performance as compared to conventional clear glass.
Different materials may be added to the glass in order to provide the desired color and spectral performance. For example, iron, cobalt, nickel, selenium, cerium, and titanium, to name a few, are typically added to provide the desired color composition. As materials are added to change color and enhance solar performance, care must be taken to maintain the visible light transmittance and color required for that particular application. It should also be remembered that changing the thickness of the glass affects these spectral properties so that a particular composition which has acceptable color and performance at a particular thickness may not be acceptable at a different thickness. The conversion of these properties to different thickness of glass is discussed in U.S. Pat. No. 4,792,536 to Pecoraro, et al. at column 15 line 59 through column 16 line 7.
One particular blue composition that provides superior spectral performance is disclosed in U.S. Pat. No. 4,792,536 to Pecoraro, et al. Commercial products covered by this patent were sold by PPG Industries, Inc. under the SOLEXTRA(copyright) and AZURLITE(copyright) trademarks. This glass incorporates a moderate amount of iron in the composition and has a relatively large portion of the glass in the ferrous state, expressed as FeO. In particular, the glass composition includes a basic soda-lime-silica composition and further includes 0.45 to 1 weight percent total iron (expressed as Fe2O3). At least 35 percent of the total iron is in the ferrous state. The dominant wavelength of these glasses range from about 486 to 489 nanometers (xe2x80x9cnmxe2x80x9d) and excitation purity ranges from about 8 to 14 percent. From a processing standpoint, producing the glass disclosed in this patent with a high ratio of ferrous iron to total iron may require additional processing considerations not typically associated with conventional glass melting operations, as are well known in the art.
Horizontal elongated continuous melting furnaces with several different operational sections for the molten pool of glass leads to forming operations at one end to produce flat glass from batch materials and optionally cullet generally introduced to the furnace at the opposite elongated end. The furnaces melt the batch and any cullet through the application of heat to form a pool of molten glass that flows through the furnace for refining and conditioning into formable melted glass withdrawn for forming flat glass. Furnaces of various design have various heat sources such as overhead fossil fuel fired burners and/or electric heat sources. Fossil-fueled furnaces include regenerative furnaces and recuperative furnaces, and electric furnaces include those as illustrated in U.S. Pat. No. 2,225,616 and U.S. Pat. No. 2,225,617. There are fossil fuel-fired furnaces which include electric-boosting electrodes as shown in U.S. Pat. Nos. 2,397,852, 2,600,490, 2,636,914 and 2,780,891. In general, when both electricity and fossil fuels have been used to provide heat to the same glassmaking furnace, the fossil fuels have been employed to melt glass batch in the region of the furnace where batch is advancing freely through the furnace from its charging kilns. Electrodes have been positioned at various locations in the furnace in order to assist in melting batch, to heat molten glass beyond the region of unmelted batch, and to strengthen the convective flow, known as the xe2x80x9cspring zonexe2x80x9d flow, within the molten glass as shown in U.S. Pat. Nos. 2,512,761, 2,636,914 and Canadian Pat. No. 634,629, and the like.
The solar absorbing glasses, like those with a high redox value of greater than 0.38 with the higher amounts of the reduced form of iron, ferrous iron, and especially with high iron concentration of greater than 0.45 to 1.5, pose challenges in melting and refining and conditioning in horizontal continuous furnaces. The melting and refining and/or conditioning whether in one, an adjoining, or separate sections or zones of the furnace for the refining and conditioning operations can each experience similar and different upsets and impediments in producing these glasses. Although the higher amount of ferrous iron results in glasses that absorb infrared energy, this higher amount of ferrous iron also provides for low effective thermal conductivity in the glass melt. This leads to a more pronounced temperature gradient in the depth of the pool of melted glass than is encountered with lower redox glass melts. Consequences of such a situation depending on the location within the furnace can result in a much lower tolerance to both changes in temperature and depth of the melted glass pool. Results of these situations can range from bubble defects and/or ream defects or imperfections in the flat glass product to premature glassy formations from the molten glass in the furnace. These defects include gradations for each. For instance the bubble defects range from seed defects to partial and to closed bubble defects that can occur at different locations in the depth of the glass product such as top and bottom bubble defects. The ream defect relates to Rayleigh instability as described in U.S. Pat. No. 3,836,349.
Also one way of achieving a higher redox value in glass is the formation of ferrous iron during melting and/or refining of the glass melt for the solar absorbing glasses through the use of reductants and the removal of oxidizers from the batch and/or cullet ingredients. For the refining of a glass melt in horizontal continuous furnaces having overhead fossil fuel firing, chemical fining agents such as sulfur-containing fining agents generally are used. These fining agents assist in the removal or resolution of gaseous inclusions from or in the melt. The dilemma is that the sulfur-containing fining agents are oxidizers which depending on their amounts can shift the balance of the amount of reduced iron, ferrous iron, to oxidized iron, ferric iron, in the glass melt and product creating difficulties in achieving higher redox values. The reduction or removal of sulfur-containing oxidizing fining agents from the batch ingredients can impede the fining operation of the melted glass. A resolution is needed for the use of sulfur-containing fining agents to obtain high quality glass with minimum defects while at the same time limiting the oxidation of iron to achieve higher redox values for the glass.
The favorable acceptance of the SOLEXTRA product makes it advantageous to produce a type of glass having a similar color and enhanced spectral performance using conventional glass melting furnaces. Also the market may find glasses of other colors such as green, blue-green, green gray, blue grey and grey with good spectral properties to be of value. Also the production of solar performing glasses of these other colors with their higher ferrous content could benefit from more facile production processes in more conventional melting furnaces in a manner similar to that for a type of glass like the SOLEXTRA glass.
An object of the present invention is to produce quality tinted or colored glass with superior spectral performance from a horizontal continuous furnace having overhead fossil fuel fired burners.
2B. Patents of Interest
U.S. Pat. No. 3,652,303 to Janakirama Rao discloses a blue, heat absorbing glass which incorporates low amounts of iron and uses tin to convert and retain a significant portion of the iron in the ferrous state, and in particular more than 80% of the iron is retained in the ferrous state.
U.S. Pat. Nos. 4,866,010 and 5,070,048 to Boulos, et al. disclose blue glass compositions with a colorant portion iron and cobalt and further including nickel and/or selenium. The glasses have a dominant wavelength of 482 nanometers (nm) xc2x11 nm and a color purity of 13% xc2x11%.
U.S. Pat. Nos. 5,013,487 and 5,069,826 to Cheng disclose blue colored glass compositions which include iron, titanium, tin and zinc as colorants. The glasses have a dominant wavelength ranging from 485 to 494 nm and a color purity of 5 to 9%.
U.S. Pat. No. 5,344,798 to Morimoto, et al. discloses a blue glass composition which includes iron, cerium, titanium, zinc, cobalt and manganese. These glasses have a dominant wavelength of 495 to 505 nm and a color purity of 5 to 9%.
The aforementioned object and resolution of the dilemma is accomplished by the type of glass and method of making it provided by the present invention. The solar absorbing glass has a high performance ultraviolet and/or infrared absorbing soda lime float glass composition. The glass has a higher iron content of greater than 0.4 and up to 2 weight percent of the glass composition, and has a redox ratio in the range of greater than 0.38 to about 0.65. The retained SO3 content where sulfur is in +6 oxidation state is less than 0.18 weight percent of the glass. This glass is essentially free of amber coloration from inorganic polysulfides, and is produced from melting and refining under controlled temperature, residence time, and environmental conditions where the batch composition comprises: soda lime silica glass forming batch materials, an essential solar radiation absorbing and colorant forming portion, at least one sulfur-containing fining agent and at least one reducing agent in a controlled ratio of one to the other. The relatively low level of dissolved sulfur-containing fining agent in the melt is compensated for by controlling the time-temperature integral of melting and refining which is greater than that for a lower redox glass. Optionally high iron cullet is added in an amount in the range of around 25 to around 75 weight percent of the batch. The controlled ratio of at least one sulfur-containing fining agent to reducing agent can be in the range of about 2.4 (pref 3.5) to about 4.5 when salt cake is the at least one sulfur-containing fining agent and coal is the reducing agent and the time temperature integral is modified by one or more of the following controlled melter and refiner operations:
For the melter:
a) a higher temperature of melting than for lower redox (less than 0.38) glasses to assist in containing batch melting more upstream in the melter for instance through a different burner firing pattern and aggressive use of bubblers;
b) use of cullet with the batch for melting where the cullet is present in the range of weight percent for the batch from 20 to 80 percent and the cullet can be selected from high redox cullet with a redox ratio from 0.5 to 0.7 and from clear cullet and mixtures thereof, where lower amounts of cullet utilized with the batch usually involve cullet with a higher redox ratio;
c) reduction of the temperature of glass melt entering the refiner portion of the furnace by up to 70xc2x0 F. (21xc2x0 C.) for instance with utilization of glass cullet with a high redox ratio in the range of 0.5 to 0.7 with the batch ingredients of 20 to 80 weight percent;
d) maintaining appropriate temperature gradient in the melt for strong convection currents for improved heat transfer and homogeneity of the melted glass;
e) increase residence time or melting temperature in the melter when the batch feed ingredients to the melter have lower amounts, around 20 weight percent of clear cullet, as opposed to the use of greater than 40 percent high redox cullet;
f) maintain the upstream melter bottom temperature at least above the liquidus point of the glass composition through electrode heating of the bottom or aggressive use of bubblers to assist in heat transfer through the pool of melted glass;
f) avoid defect density of bubbles by 1) major reduction of glass temperature entering the waist or the beginning of the refiner if a waist is not present, or 2) providing an oxidizing atmosphere above the glass in the downtank port before the waist or the beginning of the refiner, or
For the refiner:
a) provide front end firing at a temperature less than that which causes reboil bubbles to retard the surface glass from cooling and increasing in density and sinking in the pool of melted glass;
b) provide controlled cooling in the refiner to obtain more laminar flow of the melt to avoid ream-induced distortion from the IR absorption properties of the reduced glass through an appropriate cooling configuration from the waist or the beginning of the refiner if no waist is present through the forming canal or entrance in order to avoid the formation of Rayleigh roll cells, (one approach is to cool the under glass temperature by submerged coolers),
c) maintain a temperature of the refined melted glass removed for formation into flat glass with two opposing major surfaces and a consistent thickness between them in the range of 1 to 12 millimeters (xe2x80x9cmmxe2x80x9d) such that the downstream refiner bottom temperatures are greater than the liquidus temperature for the glass;
d) optionally utilize one or more longitudinal heat extracting members positioned transverse to the direction of flow of the pool of molten glass to conserve melter temperature by restricting flow at the surface and/or extracting heat in the pool of melted glass in the refining section at selected discrete regions of the pool;
One aspect of the present invention provides a blue colored glass using a standard soda-lime-silica glass base composition and additionally iron and cobalt, and optionally chromium, as the essential solar radiation absorbing material and colorant portion. In particular, the blue colored glass includes as the essential solar radiation absorbing and colorant portion about 0.40 to 1.0 wt % total iron, preferably about 0.50 to 0.75 wt %, about 1 to 40 PPM CoO, preferably about 4 to 20 PPM, and 0 to 100 PPM Cr2O3. In one particular embodiment the redox ratio for the glass of the present invention has between about 0.50 to 0.55, a luminous transmittance of at least 55 percent and a color characterized by a dominant wavelength of 485 to 489 nanometers and an excitation purity of about 3 to 18 percent. In another embodiment of the invention, the glass has a luminous transmittance of at least 65 percent at a thickness of about 0.154 inches (3.9 mm) and a color characterized by a dominant wavelength of 485 to 492 nanometers and an excitation purity of about 3 to 18 percent.
The method of the present invention involves: feeding glass-making ingredients for high redox soda lime silica glass with essential colorant and solar radiation absorbing formation materials having one or more sulfur-containing fining agents and one or more carbonaceous reducing agents wherein the ratio of the former to the latter is in the range of about 3.5 to about 4.5 along with cullet into a glass melting furnace, melting the ingredients at higher temperatures than low redox glass melt less than 0.38, controlling the time temperature integral with controlled cooling from the hot spot to assist refining in the refiner and to minimized the formation of ream imperfections in the glass and remove at least a portion of the refined melted glass for formation of flat glass articles with two opposing major surfaces and a consistent thickness ranging from around 1 to around 12 mm between these major surfaces. The furnace has melting and refining sections wherein a large portion of the heat is applied to the ingredients from over the melt by fossil fuel fired burners to melt the same and form a pool of molten glass at least a portion of which flows from the melting section of the furnace to the refining section thereof to be refined and at least a portion of the refined glass flows to exit end of the furnace where molten glass is continuously removed to a flat glass forming operation. Temperature gradients exist in the pool of molten glass in the refining section from the top surface to the bottom surface of the pool of molten glass wherein the temperature gradients cause (1) convection flow of a portion of the pool of molten glass along a generally circuitous path having a downstream direction adjacent the top surface of the mass of molten glass in the refining section and the upstream direction adjacent the bottom surface of the pool of glass in the refining section; and (2) convolutions in the pool of molten glass adjacent the top surface of the pool of molten glass in the refining section. The controlled cooling is accomplished by one or more of the following steps:
heating the upper portion of the pool of melted glass as the upper surface enters the refiner;
disposing a first member or set of members for extracting heat in the pool of molten glass in the waist or front of the refining section as a discrete region along the circuitous path in the upstream flow transverse to the direction of flow of the pool of molten glass;
extracting heat by way of the first heat extracting member from each of the discrete regions as the molten glass moves past the region wherein the rate of flow of the pool of glass as it moves past the regions is reduced;
disposing second member or set of members for extracting heat from under the surface of the molten pool of glass in the refiner;
extracting heat by way of the second heat extracting member from the pool of molten glass as the pool of the molten glass moves along the upstream portion of the circuitous path;
Performing the extracting steps to extract heat from the pool of glass as it moves along the circuitous path to alter the temperature gradients such that the convolutions in the pool of molten glass are minimized.